The preparation of polyisocyanates having blocked isocyanate groups and their use in combination with polyhydroxyl compounds for the production of polyurethane resins and in particular for the production of stoving lacquers is already known (see e.g. Kunststoff-Handbuch, Volume VII, Polyurethane, 1966, Carl-Hanser-Verlag, Munich, pages 17 and 24). The blocking agents used in practice, apart from phenols, are mainly oximes and .epsilon.-caprolactam. In addition to ensuring sufficient storage stability of the combination of blocked polyisocyanate and polyhydroxyl compound at room temperature, these blocking agents also ensure sufficient reactivity of the combinations at elevated temperatures.
The above mentioned combinations already known in the art are in practice used in the form of relatively dilute solutions having a solids content of about 20-50% by weight. Attempts to introduce solvent-free or low solvent stoving lacquers have failed in practice due to processing difficulties, because there is usually a close correlation between solids content, viscosity and flow. On the other hand, there are strict limits to the possibility of lowering the viscosity by increasing the temperature employed for processing the systems because a rise in temperature normally eliminates the latent blocking of the isocyanate groups, with the result that the lacquers undergo an undesirable increase in viscosity and finally may even gel. Another considerable disadvantage of the above mentioned combinations known in the art, in particular the combination based on polyisocyanates blocked with phenol groups, lies in the toxicity of the phenol split off during the stoving process.
There have therefore been several attempts to replace the above mentioned blocking agents, in particular phenols, by blocking agents which are not only nontoxic but also provide for problem-free processing with solvent-free or low solvent systems.
A partial solution to this problem can be seen in the process according to German Offenlegungsschrift No. 2,404,740 which corresponds to U.S. Patent Application Ser. No. 544,620 filed Jan. 28, 1975. When the alkanol-blocked or cycloalkanol-blocked isocyanates mentioned in this publication are used in combination with the usual polyhydroxyl compounds, they give rise to low viscosity, solvent-free systems which are stable in storage even at temperatures above about 150.degree. C. The blocked polyisocyanates mentioned in the above prior publication mainly consist of clearly defined, relatively low molecular weight organic compounds which are liable to undergo partial evaporation under the conditions employed for wire lacquering and may thus result in considerable stoving losses during the lacquering process. Moreover, the alkanol-blocked and cycloalkanol-blocked polyisocyanates mentioned in the aforesaid prior publication could not be used satisfactorily in systems containing solvent because the stoving time required for evaporating off the solvents and reaching the high temperature necessary for liberating the blocked polyisocyanates is too long and results in unsatisfactory film properties in the case of the above described systems based on combinations of polyhydroxyl compounds and low molecular weight polyisocyanates blocked with alkanol or cycloalkanol.
It was, therefore, an object of the present invention to provide a method of obtaining blocked polyisocyanates which would not have the disadvantages described above but could be used universally, i.e. both in solvent-free systems and in systems containing solvents, and could result in satisfactory stoving lacquers, in particular wire lacquers, without at the same time contaminating the environment with toxic blocking agents.
It was surprisingly found that this problem could be solved by the process according to the invention which is described below, in which oligourethanes having isocyanate end groups blocked with alkanol or cycloalkanol are obtained.